Ultra-wideband (UWB) radio systems transmit and receive communication signals as modulated impulses. The duration of the modulated impulses is typically very short and is of the order of a few fractions of a nanosecond (ns). This allows the modulated impulses to have frequency ranges that are extremely broad, typically of a few gigahertz (GHz). The broad frequency ranges of the UWB radio systems are therefore distinctly different from conventional narrow-band radio systems. This distinction of the UWB radio systems require a unique set of regulations implemented by a regulatory body specifically for communication systems that are based on UWB technology. The regulations limit the radiated power levels and signal spectra of the UWB radio systems in order to facilitate undue interference to the conventional narrow-band radio systems which occupy a part of the frequency spectrum of the UWB radio systems.
One such regulation, as stipulated by the US Federal Communication Commission (FCC), requires that the emission levels and spectra of the radiated pulses of a UWB radio system to have an effective isotropic radiated power (EIRP) below −41.3 dBm/MHz for a 10 dB bandwidth that covers a frequency range from 3.1 to 10.6 GHz. This regulation defines a spectral limit mask for all UWB radio systems.
Previous studies have shown that emission and reception patterns of a UWB radio system are significantly affected by its antenna characteristics. Therefore, the emission and reception patterns of the UWB radio system are typically modified to conform to FCC emission regulation on the limit mask by appropriately designing the antenna characteristics.
Besides meeting the limit mask regulation, antennas of a UWB radio system should be designed to fulfill a number of requirements. Firstly, the UWB radio system has a bandwidth that is as broad and well-matched as possible for achieving broadband capability and attaining high system efficiency. Secondly, operating power of the UWB radio system is as low as possible for attaining high power efficiency. Thirdly, the UWB radio system has a linearised phase transfer response for providing minimal signal distortion. Finally, the UWB radio system generates radiated pulses with maximum power in a desired direction.
Numerous attempts have been made to fulfill the requirements through various designs of antennas for the UWB radio system. More notable examples are transverse electromagnetic mode (TEM) horns and self-supplemental antennas, such as spiral antennas. Both types of antennas feature very broad and well-matched bandwidths. However, pulses generated by both types of antennas are distorted and suffer from dispersion due to frequency-dependant changes in their respective phase centers.
Bi-conical and disk-conical antennas have less distortion and have relatively stable phase centers for achieving a broad and well-matched bandwidth. This is because resistive loadings are used to eliminate reflection of radiated pulses occurring at transmission ends of both antennas. However, both antennas are bulky in size and are thus unsuitable for small and portable UWB devices.
In conjunction with the abovementioned requirements for a UWB radio system, another important consideration for designing a UWB antenna is the preclusion of interference to conventional in-band or out-band radio systems. The UWB antenna is required to function as an efficient radiator that precludes interference to in-band systems such as W-LAN operating at 5.2 or 5.8 GHz or out-band systems operating at 0.99 to 3.1 GHz.
Further attempts have been made to provide UWB antennas with broadband capability and compliancy with requirements for non-interference with existing in-band and out-band radio systems. In U.S. Pat. No. 6,437,756, Schantz teaches a notched planar monopole to attain band-notched characteristics with a well-matched bandwidth for a voltage standing wave ratio (VSWR) of less than 2:1. However, the well-matched bandwidth is not sufficiently broad for UWB applications.
In U.S. patent application 2003/0090436 A1, a shorted planar monopole having a shorting pin at the bottom of the monopole is proposed by Schantz et al. for size reduction. However, in order to maintain radiation efficiency, the shorting pin and a feed to the monopole are separated far apart, thus rendering the lateral size of the monopole large. The bandwidth of the monopole is also not broad enough for UWB applications.
In U.S. patent application 2002/0122010, McCorkle proposes using a small annular planar monopole to achieve a broad and well-matched bandwidth. However, the annular planar monopole does not exhibit band-notched characteristics for the fulfillment for non-interference with existing in-band and out-band radio systems.
There is therefore a need for an antenna for a UWB radio system which is dimensionally small and for improving system efficiency and reducing interference with existing radio systems.